There's nothing quite like being in the throws of a good horror-- whether book, TV, or movie -- and having all the creepies ruined with a terrible ending. In extreme cases, the disappointment is so great that all the amazing potential you felt for the book or film seems to magically disappear, ruining its chances at making itself a new favorite.
As a writer I find the idea of hooking an audience like a boss-wizard, only to let them down in the end, just as nauseating as if there had been no hook at all. Pop, fizzzzzz.
And with horror, where plot holes are something that the audience almost goes out of their way to look for, the power is in the details. One small slip and the final product you had in your mind can slip through your fingers like that. (Because let's be honest, as writers we definitely put that kind of pressure on ourselves.)
To the audience, the ending may only appear as a single, interchangeable component to the formula. However, as writers we know that it's really a product of all the other components combined. These are just some general patterns I've noticed in horror that all lead to a less-than-satisfying ending.
A) Starting Out Great And Tapering Off:
Picture this. You get a brilliant new idea for a WIP, so bright and vivid that you need to start it now now now! So you're off. You pound out scene after scene after scene with intense satisfaction, just knowing that it's coming out like you planned. In fact, as you're re-reading, you're guessing that your lukewarm ending (which was likely not included in your original vision,) will totally suffice, since the rest is just so sparkly.
WRONGETH! You've gone this far to build a story that was awesome from page one...and that story deserves to go strong all the way. Go out of your way to make sure things are consistent and vivid to the end, and you'll have a happy audience.
B) Writing Yourself Into a Hole
Sometimes horror inspiration presents itself as bursts of random scary bits you've come up with on a stroke of genius, which doesn't exactly include a planned out, cohesive story. If you decide to go off of those lovely terrifying visions alone and hope that the plot will fall into place, you had better be ready for some major reworking/rewriting later. And the danger for the ending is this: you may have set your readers' hopes a little too high, as far as what you've concocted so far. If you're flying by the seat of your pants and write yourself into a hole, the audience will be able to identify the cop-out and will come away feeling annoyed.
That is not to say that you shouldn't take chances as you're writing the book. Author Gretchen McNeil recently threw out a fun fact about her book TEN on Twitter, in which she stated that she in fact didn't know exactly who the killer(s) was until she sat down to write the final scene. On the surface it seems a little too gutsy, but the reality of it is so much smarter.
McNeil had narrowed it down to three candidates, and therefore was probably able to have a lot of fun and allow herself to get caught in the throws of the delicious horror mystery aspect while she was writing. If she herself didn't know until the end, you had better believe that the reader probably won't either.
C) Revealing vs. Explaining: the 'Show vs. Tell' of Horror
One of the simplest and greatest pieces of writing advice that we've all heard a billion times by now is to show and not tell. Writing this way will strengthen your manuscript throughout and give it much more relatable emotion for the reader to identify with.
When it comes to horror, your ending needs to be able to satisfy to the reader without spilling all the beans. If you've got a Big Bad with complicated lore, do not do not do not spell it entirely out for the reader, and especially at the end of the book. This will have a terrible impact in the fact that all the mystery will have been taken away, something really needed to leave the audience with a lingering fear after they have finished the last page.
That's not to say that you should leave them with no information at all. Instead of having an explanation at the end of your horror, have a reveal. Pull back the curtain just enough for the reader to peek inside and come to their own conclusions. You can nudge them in certain directions, but be gentle: the horror audience does NOT like to be told what they're afraid of, they like to experience it.
D) Doing Something Too Unexpected
I'm guessing that this isn't just me, but one of my biggest fears as a horror writer is to write something predictable. Not that I fancy myself as a huge twist genius, but if there happens to be a surprise at the end, well, I'd like the reader to be surprised. And sometimes while you're writing the book, it can be tempting to lead the reader away from the inevitable conclusion so that they'll be shocked later on.
However, this can be taken to an extreme, and by the time the reveal comes along the reader could end up just scratching their heads and going, "HUH!?" The mystery aspect of the reveal still needs to be strong, but having it be TOO strong will just leave you with a book that doesn't make any sense, to anyone besides you anyway. No bueno.
E) On the Flip Side- Doing Something Not Unexpected Enough
If you're aware of the ending, it can feel really easy to drop what you perceive to be subtle hints here and there to help build up to the finale. Beware, though: if the clues you are dropping stick out from the details that surround them, the audience will spot them and likely put two and two together, coming out disappointed in the end. Being able to spot a horror ending from a mile away is almost painful, because the distracting feeling of "I'm right aren't I? Yes this supports my theory! I KNOW HOW THIS THING ENDS I BET" totally takes away from the roots of the horror reading experience, which is to feel afraid here and there.
So when you drop those subtle clues, make sure they blend in with the situation at hand, otherwise they'll stick out like a sore thumb.